The present invention relates generally to systems and methods for performing noninvasive surgical procedures using focused ultrasound, and more particularly to systems and methods for controlling distribution of acoustic energy in the vicinity of a focal point, for example, to create broad, uniform necrosis volumes around the focal point using a focused ultrasound transducer array.
High intensity focused acoustic waves, such as ultrasonic waves (acoustic waves with a frequency greater than about 20 kilohertz), may be used to therapeutically treat internal tissue regions within a patient. For example, ultrasonic waves may be used to ablate tumors, thereby obviating the need for invasive surgery. For this purpose, piezoelectric transducers that may be driven by electric signals to produce ultrasonic energy have been suggested that may be placed external to the patient but in generally close proximity to the tissue to be ablated. The transducer is geometrically shaped and positioned such that the ultrasonic energy is focused at a xe2x80x9cfocal zonexe2x80x9d corresponding to a target tissue region within the patient, heating the target tissue region until the tissue is necrosed. The transducer may be sequentially focused and activated at a number of focal zones in close proximity to one another. This series of sonications may be used to cause coagulation necrosis of an entire tissue structure, such as a tumor, of a desired size and shape.
A spherical cap transducer array, such as that disclosed in U.S. Pat. No. 4,865,042 issued to Umemura et al., has been suggested for this purpose. This spherical cap transducer array includes a plurality of concentric rings disposed on a curved surface having a radius of curvature defining a portion of a sphere. The concentric rings generally have equal surface areas and may also be divided circumferentially into a plurality of curved transducer elements or sectors, creating a tiling of the transducer face. The transducer elements are driven by radio frequency (RF) electrical signals at a single frequency offset in phase and amplitude. In particular, the phase and amplitude of the respective drive signals may be controlled so as to focus the emitted ultrasonic energy at a desired xe2x80x9cfocal distance,xe2x80x9d i.e., the distance from the transducer to the center of the focal zone and provide a desired energy level in the target tissue region.
In addition, the phase of the respective drive signals to each of the sectors may be controlled to create a desired size and shape for the focal zone. For example, if each of the sectors are driven with respective drive signals that are in phase with one another (xe2x80x9cmode 0xe2x80x9d), the ultrasonic energy may be focused substantially at a relatively narrow focal zone.
Alternatively, the sectors may be driven with respective drive signals that are in a predetermined phase relationship with one another (referred to, in Umemura et al., as xe2x80x9cmode nxe2x80x9d). This results in a focal zone that includes a plurality of 2n zones disposed about an annulus, i.e., generally defining an annular shape, creating a wider focus that causes necrosis of a larger tissue region within a focal plane intersecting the focal zone. One problem with such an annular focal zone, however, is that it may result in a xe2x80x9chole,xe2x80x9d i.e., a region within the annular focal zone that is not successfully necrosed, despite the necrosis of the surrounding tissue within the annular focal zone. This may be particularly problematic in applications where a relatively short sonication time is used such that the necrosed volume approximately matches the shape of the annular focal zone. In such applications, thermal diffusion may not play a central role in shaping the necrosed volume.
A proposed solution to this problem is temporal switching, suggested by D. R. Daum and K. Hynynen in xe2x80x9cThermal Dose Optimization Via Temporal Switching in Ultrasound Surgeryxe2x80x9d, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 45, no. 1, pages 208-215 (January 1998). Temporal switching involves sequentially switching a transducer between various modes during a sonication to provide a more xe2x80x9cflatxe2x80x9d heating profile around the focal plane.
Accordingly, it would be desirable to provide systems and methods for treating a tissue region using focused ultrasound that provide a more broad, uniform volume of treated tissue and/or that reduce the risk of leaving untreated tissue within a target tissue region being treated, for example, without the complications of temporal switching.
The present invention is directed to systems and methods for performing a therapeutic procedure using focused ultrasound, and more particularly to systems and methods for controlling distribution of acoustic energy near or around a focal point, for example, to create broad, uniform necrosis volumes around the focal point.
In accordance with one aspect of the present invention, a system is provided that includes a transducer array including a plurality of transducer elements disposed about a central axis. Each transducer element has an angular position in a circumferential direction about the central axis. Drive circuitry is coupled to the transducer elements, the drive circuitry configured for providing respective drive signals to the transducer elements. A controller is coupled to the drive circuitry, the controller configured for controlling the drive circuitry to drive the transducer elements with respective drive signals that have phase shift values based upon the angular position of each respective transducer element.
The phase shift values are preferably based upon an oscillation function that oscillates between minimal and maximal phase shift values, and that repeats itself a predetermined number of times about the circumferential direction. The oscillation function has an amplitude defined by the difference between the maximal and minimal phase shift values that is not one hundred eighty (180) degrees or an integer multiple thereof. In a preferred embodiment, the oscillation function is a step function that alternates between the minimal and maximal phase shift values for driving alternate sectors about the circumferential direction of the transducer array. Alternatively, the oscillation function may approximate a sine wave or other function oscillating between the minimal and maximal phase shift values.
Preferably, the controller is configured for controlling the phase shift values such that a first focal zone is created that is located on the central axis, and a second off-axis focal zone is created around the first focal zone. More preferably, the controller is configured for controlling the phase shift values to optimally distribute acoustic energy between the first focal zone and the second focal zone in a desired manner. The controller may be also be configured for selecting the predetermined number of oscillations to adjust a radius of the second focal zone.
In addition, each of the transducer elements may have a radial position with respect to the central axis, and the minimal and maximal phase shift values, and consequently the phase shift values of respective transducer elements, may be shifted by a predetermined phase shift based upon their respective radial positions to adjust a focal distance of the focal zones of the transducer array.
In accordance with another aspect of the present invention, a method for performing a therapeutic procedure in a target tissue region of a patient is provided that includes providing a transducer array including a plurality of transducer elements disposed about a central axis, each transducer element having an angular position in a circumferential direction about the central axis. The plurality of transducer elements are driven with respective drive signals, while substantially focusing ultrasonic energy produced by the plurality of transducer elements at a focal region.
Phase shift values of the acoustic energy generated by each transducer element are controlled based upon an oscillation function wherein the phase shift values oscillate between minimal and maximal phase shift values based upon the angular position of the respective transducer element such that a first focal zone is generated that is located on the central axis, and a second off-axis focal zone is generated that is disposed around the first focal zone. The minimal and maximal phase shift values may be selected to adjust a relative maximum acoustic intensity of the first and second focal zones, and preferably such that the maximum acoustic intensity of the first focal zone is substantially the same as the maximum acoustic intensity of the second focal zone. Alternatively, the minimal and maximal phase shift values may be adjusted such that the acoustic energy delivered to the target tissue creates a more flat temperature profile, namely an optimal energy distribution that may not necessarily correspond to equal maximum acoustic intensities in the first and second focal zones.
The oscillation function may include a step function that alternates between the minimal and maximal phase shift values between adjacent sectors about the circumferential direction, or may approximate a sine wave or other periodic function. In addition, the oscillation function may be controlled such that the oscillation function repeats itself a predetermined number of times about the circumferential direction, for example, to provide a desired radius of the second focal zone.
In accordance with another aspect of the present invention, a method for performing a therapeutic procedure in a target tissue region of a patient using focused ultrasound is provided that includes providing a transducer including a central axis, and providing an acoustic lens between the transducer and a focal plane, the acoustic lens including a plurality of regions or segments disposed about the central axis of the transducer array. The transducer is driven with drive signals, while substantially focusing ultrasonic energy produced by the transducer at the focal region. The acoustic lens is controlled or otherwise allowed to create a phase shift in the acoustic energy emitted by the transducer and passing through respective regions or segments of the acoustic lens based upon an oscillation function that oscillates between the minimal and maximal phase shift values based upon an angular position of respective regions or segments of the acoustic lens such that a first focal zone is generated that is located on the central axis, and a second off-axis focal zone is generated that is disposed around the first focal zone. Alternatively, the lens may be built in such a manner as to enable continuous changing of the phase shift values about the central axis, i.e., based upon the angular position on the surface of the lens.
Thus, a system in accordance with the present invention may generate a focal zone that includes both an on-axis component and an off-axis component that may reduce the risk of leaving unablated tissue within a target tissue region during a sonication.
Other objects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings.